The optical constants n and k have been calculated for fused silica, three silicate glasses, and a phosphate glass from reflectance measurements made in the strong absorption region, from about 7 µ to 13 µ. Plots of the 2nkv product, which corresponds to the conductivity, as a function of frequency are used to determine the location of the resonance frequencies in the infrared.
Given W. Cleek, "The Optical Constants of Some Oxide Glasses in the Strong Absorption Region," Appl. Opt. 5, 771-775 (1966)